Study of non-genetic factors on the shape of lactation curves for milk yield, fat and protein percents of Holstein- Friesian cows under hot Mediterranean climate

SUMMARY This investigation was done to study the effect of non-genetic factors on the lactation curve of Holstein-Friesian cows reared under hot Mediterranean climate. The shape of the lactation curve of 5649 Tunisian Holstein-Friesian cows were estimated by fitting the Wilmink model to 259,776 test-day records for milk traits. Analyses were conducted for six groups edited according to the calving age of cows associated with their parity, three herd groups were formed according to the level of average milk yield from calving years and four seasons were also used. All sources of variation were significant, except the calving age-parity was not significant (P>0.05) for b and c both for fat and protein percents. The herd-calving year effect was independent of persistency for all milk traits and not significant for b (fat percent) and for b and c (protein percent). The calving season was not significant for fat persistency. Peak and lactation yield were the highest for cows calving at 4-5 years (77% in the third parity). Peak fat and protein percentage occurred at the beginning or/and at the end of lactation. Cows in different herd-calving year groups presented almost the same persistency for all Milk traits but recorded different yields at 305-days of lactation. The lactation curve of cows with the lowest initial level and a rapid decrease to the nadir point (the minimum point) presented a very high inflection especially around the nadir point resulting in an almost linear increase of the fat or protein percent to the end of lactation. This shape resulted to produce the lowest 305-day fat and protein yields. The highest peak and lactation milk yields were reached by cows that calved in winter and fall. Cows calved in the summer and spring presented a different shape of lactation curve but produced almost the same 305-day milk yield. In summer,